1. Field of the Invention
In one of its aspects, the present invention relates to a fluid treatment system. In another of its aspects, the present invention relates to a process for treating fluid.
2. Description of the Prior Art
Mixing devices are known in the art and have been used to promote fluid turbulence—for example, to improve contact between elements in the flow path. Industrial applications of mixing are widely varied, and include heat exchange, reactor engineering and non-reactive blending.
One specific area of reactor engineering where mixing has been used is in the design of fluid treatment devices, particularly fluid radiation treatment devices. A specific such fluid radiation treatment device includes ultraviolet (UV) disinfection devices for water and wastewater treatment. The performance of UV disinfection devices depends, at least in part, on providing a prescribed dose of UV radiation to all fluid elements passing through (or otherwise being treated by) the device.
The UV dose received by a fluid element is defined as the product of UV intensity and exposure time. The accumulated UV dose received by a fluid element exiting the device is the sum of the individual doses received at each position. Since the UV intensity is attenuated with the distance from the UV source, it is desirable to mix fluid elements from regions far from the UV source to regions of higher intensity nearer to the source, thereby ensuring they receive an adequate dose of UV radiation. This type of mixing is particularly desirable when the transmittance of UV radiation through the fluid being treated is low (e.g., less than about 30% per cm), causing an increase in the attenuation of UV intensity with distance from the source—this is commonly encountered in UV disinfection devices for the treatment of liquids such as wastewater.
U.S. Pat. No. 5,846,437 [Whitby et al. (Whitby)], assigned to the assignee of the present application, teaches turbulent mixing in a UV system. More specifically, Whitby teaches the use of one or more ring-shaped devices (e.g., washers) at predetermined locations on the exterior surface of each lamp unit in the system and/or ring-shaped devices upstream of each lamp unit to increase turbulent mixing of fluid passing by the lamp units. While such ring-shaped devices as taught in Whitby are useful in increasing turbulence between the lamp units, the turbulent flow of fluid tends to be of a random or non-ordered (e.g., isotropic) nature.
In many systems, such as those where the mixing zone is longitudinal with respect to the direction of fluid flow therethrough, it is desirable to have plug flow in the flow direction and effective mixing in the transverse (to flow) direction. A specific or ordered pattern of fluid flow in the mixing zone is desirable (e.g., a “particle” of fluid oscillating toward and away from the lamp as it passes longitudinally with respect thereto), which is in contrast to general mixing in all directions (i.e., in contrast to random mixing or turbulence taught by Whitby). A longitudinal vortex is an example of this type of flow pattern. Vortices can be formed actively through energy input to the fluid, such as by employing a motorized fluid impeller.
Another means of achieving vortex generation is through the use of a passive element which is designed to cause the formation of the desired flow pattern (vortex generator).
U.S. Pat. Nos. 5,696,380, 5,866,910 and 5,994,705 [all in the name Cooke et al. (Cooke)] teach a flow-through photochemical reactor. The subject reactor taught by Cooke comprises an elongate annular channel in which is disposed an elongate radiation source. The channel includes static, fluid-dynamic elements for passively inducing substantial turbulent flow within the fluid as it passes through the channel. According to Cooke, each such static, fluid-dynamic element advantageously creates a pair of “tip vortices” in the fluid flow past each element. The “tip vortices” purportedly are counter-rotating about an axis parallel to the elongate annular chamber.
U.S. Pat. No. 6,015,229 [Cormack et al. (Cormack)], assigned to the assignee of the present application, teaches a fluid mixing device. The fluid mixing device comprises a series of “delta wing” mixing elements which cause the formation of vortices thereby improving fluid mixing. A specific embodiment of such a device illustrated in Cormack is the use of “delta wing” mixing elements to cause such vortex mixing between UV radiation sources in an array of such sources. This creates the potential for increasing distance between adjacent UV radiation sources in the array which, in turn, allows for a reduction in hydraulic head loss of the fluid flow through a UV disinfection system comprising the fluid mixing device.
U.S. Pat. No. 7,166,850 [Brunet et al. (Brunet)] teaches a fluid treatment device having at least one mixing element oriented in a manner to achieve improved mixing of the fluid. The fluid mixing device comprises at least one mixing element and is designed to create at least one vortex adjacent to a surface of the mixing device which is downstream of the mixing element. The mixing element comprises a centroid and is oriented in the fluid flow in a manner such that a first normal located at the centroid of the mixing element intersects a second normal emanating from the surface at the centroid of the mixing element such that the first normal, the second normal and the direction of fluid flow are in a non-planar relationship—see FIG. 2 of Brunet. This novel orientation of the mixing element results in improved fluid mixing. For example when the fluid mixing device is employed in a fluid treatment system such as UV disinfection system the improved fluid mixing is manifested in an improvement of UV dose delivery of the system. Additionally, in various preferred embodiments of the fluid mixing device taught by Brunet, such improved fluid mixing is accompanied by a reduction in hydraulic head loss of fluid passing through the system.
Despite the advances in the art made by Cooke, Cormack and Brunet, there is still room for improvement.
The systems taught by Cooke, Cormack and Brunet operate on the same general principle, namely that fluid is subject to some form of turbulence as it travels in a direction substantially parallel to the longitudinal axis of the elongate radiation source. The various mixing elements taught by Cooke, Cormack and Brunet create vortices and the like of varying degrees to optimize exposure of the fluid to radiation as the fluid travels in a direction substantially parallel to the longitudinal axis of the elongate radiation source. Unfortunately, for fluids having very low transmittance (e.g., transmittance less than 30% and as low as 5% or less), even with these enhanced mixing approaches, the fluid may not receive sufficient radiation to result in prescribed disinfection thereof.